drm/dp_mst: Simplify error path in drm_dp_mst_handle_down_rep()

Simplify the error return path in drm_dp_mst_handle_down_rep(),
preparing for the next patch.

While at it use reset_msg_rx_state() instead of open-coding it.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241203160223.2926014-4-imre.deak@intel.com